Concrete conductivity

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wbrown66

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How conductive is concrete? For some reason I always thought that concrete was non-conductive, but apparently I am wrong. My thought is that 20 ft or 1000 ft of ufer or rebar perfectly encased in 2 inches of concrete should by thoery never touch the ground/earth.
 
Conductive enough to make Table 110.26(A)(1).:grin:

Condition 2 — Exposed live parts on one side of the working space and grounded parts on the other side of the working space. Concrete, brick, or tile walls shall be considered as grounded.
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In this link, Zog posted some great info.

Dry concrete above grade 1-5 M ohms,
Dry concrete on grade 0.2-1 M ohms,
Wet concrete on grade 1-5 k ohms.

Of course resistance means nothing without an area / cross-section. In these examples I assume that the area is about equal to the contact area of an average boot sole.
 
Concrete is definitely conductive. One time I was sitting in a crawl space making a splice and I got blasted from a conductor that I thought was dead. Shock went from my hand right down through my butt and this place didn't even have a CEE.
 
Volta said:
In this link, Zog posted some great info.

Dry concrete above grade 1-5 M ohms,
Dry concrete on grade 0.2-1 M ohms,
Wet concrete on grade 1-5 k ohms.

Of course resistance means nothing without an area / cross-section. In these examples I assume that the area is about equal to the contact area of an average boot sole.
Click to expand...

Hey, thanks for saving me the work :) Yes, it is based off the area of a boot sole. I forget the sq-cm basis but I can look it up quickly if anyone cares.
 
If you have a panel in a room with the concrete on grade, take volt meter. Put one prod on the concrete floor, and the other prod on the bus of the panel. You should read very close to the actual reading of voltage at the panel.

Concrete that is in contact with the earth is a very good conductor.
 
Concrete is such a good conductor, it is the only acceptable ground electrode method the military will allow in bomb storage vaults and aircraft hangers.

Herbert G. Ufer invented what is known as A Ufer Ground during WWII. Today the NEC accepts a variant of the Ufer ground we call concrete encased ground electrode.

I have designed built quite a few of them in data centers and telephone switching offices, and in most cases the impedance is so low (less than 1 ohm) it cannot be accurately measured.
 
Concrete conductivity

Concrete is slightly alkaline (base) which is defined as OH- in solution. Anything acid (H+) or alkaline (OH-) or ionized has charge carriers in solution (in the solid).

Having an unbalanced charge, ions are hungry to give or receive another charge carrier making them conductive. Insulators have their outer valence shells completely filled and are very difficult to make share or give up a charge carrier.

This property of concrete being slightly alkaline causes steel to become passive with regard to corrosion when steel is embedded in concrete.
 
dereckbc said:
Concrete is such a good conductor, it is the only acceptable ground electrode method the military will allow in bomb storage vaults and aircraft hangers.

Herbert G. Ufer invented what is known as A Ufer Ground during WWII. Today the NEC accepts a variant of the Ufer ground we call concrete encased ground electrode.

I have designed built quite a few of them in data centers and telephone switching offices, and in most cases the impedance is so low (less than 1 ohm) it cannot be accurately measured.
Click to expand...

I read that old Herbert spent years testing his uffer ground theory in the desert were he was doing electrical work they had just sand 40 feet deep grounds rods did not work and the deeper they went with the rods was so hard they could not drive them into the rock hard bed of the desert floor so thats when he came up with the concrete encasement its always cool and damp and it takes 100 years to cure its the best ground your footer gets wet when it rains so its always a conductor . some say its better than a rod .
 
Thanks

Thanks

Thanks to everyone for replying. The reason I asked is that i took and ohm meter to concrete at work (inside in a dry area) and it would not register on the 200 ohm scale. It makes sense that the concrete closer to the earth and in direct contact with the earth will have a much higher moisture content and therefore, more conductive.
 
wbrown66 said:
Thanks to everyone for replying. The reason I asked is that i took and ohm meter to concrete at work (inside in a dry area) and it would not register on the 200 ohm scale. It makes sense that the concrete closer to the earth and in direct contact with the earth will have a much higher moisture content and therefore, more conductive.
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Well you can not test a ground connection even concrete encased with a ohmmeter you need a three point tester current and volts must be used and at a frequency not at 60 cycles,

If you test with a ohmmeter on a ground rod it will also be high in resistance .http://www.dranetz-bmi.com/pdf/groundprinciples.pdf
 
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Volta said:
In this link, Zog posted some great info.

Dry concrete above grade 1-5 M ohms,
Dry concrete on grade 0.2-1 M ohms,
Wet concrete on grade 1-5 k ohms.

Of course resistance means nothing without an area / cross-section. In these examples I assume that the area is about equal to the contact area of an average boot sole.
Click to expand...

I took over a fire alarm system install.
The previous installer had shot (powder actuated) clips into the concrete ceiling and ty-wrapped the twisted pair shielded cable to the clips.

I recieved ground fault errors in the fire alarm control panel. Multi Meter read approx. 10 M-ohms between the shield and ground.

Turns out that the ty-wraps were pulled so tight that the metal clips had pierced the outer sheath and made contact with the foil causing the foil to ground out through the concrete.

That was when I found out how conductive concrete is and how sensitive the eletronig fire alarm control panel was.

We had to go back to every clip and isolate the cable from the metal to clear the faults.
 
Pierre C Belarge said:
If you have a panel in a room with the concrete on grade, take volt meter. Put one prod on the concrete floor, and the other prod on the bus of the panel. You should read very close to the actual reading of voltage at the panel.

Concrete that is in contact with the earth is a very good conductor.
Click to expand...

Prod=Probe/test lead?
 
This might be a little off topic...but here goes; Is concrete conductive enough to replace a counterpoise? Here's the situation, an aircraft parking ramp, slightly less than 50 acres and 14" concrete with multiple static grounding points bonded by the counterpoise. I say error on the side of caution and install the counterpoise, because in the slight chance that a difference of potential can appear...the results can be catastrophic. Not likely but possible. I have been given the counter argument similar to those above, to save a few grand. Does anybody have any insight or advice on this?
 
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